O.K. Now, one more thing and please don't take offense. I have the impression you may be inexperienced on MBs.

Admittedly, I do not have the same model as you but almost all of the gas Benz do NOT use the threaded-on cap at the top of the spark plug. The plug connector is made to grip directly onto the threads. If you have the caps on the plugs, unscrew and discard them and you will find the connectors will fit easily and securely with a little push.

It is possible you have damaged the plug connectors by pulling on the wires instead of directly on the connectors. A simple pliers-like tool made for pulling spark plug connectors is available a very low cost at any chain auto parts store.

Let us know how you make out since it adds to the data base.